1. Field of the Invention
The present invention relates to an aliphatic polyester resin composition obtained by mixing a filler with aliphatic polyester in practice having substantially a high molecular weight, that is, having a number-average molecular weight of at least 10,000, which composition has a low combustion heat and biodegradability by microorganisms, and further has excellent heat stability and mechanical strength. The present invention also relates to a sheet obtained by processing such a composition and fabricated articles thereof.
2. Discussion of the Background
Conventionally, it is no exaggeration to say that high molecular weight polyesters (referring to polyesters having number-average molecular weights of at least 10,000) generally used for films, fibers and the like are limited to polyethylene terephthalate which is a condensate of terephthalic acid (including dimethyl terephthalate) and ethylene glycol or polybutylene terephthalate which is made from terephthalic acid and butyleneglycol.
In some examples 2,6-naphthalenedicarboxylic acid has been used instead of terephthalic acid, however, there has not been any example at all in which a polyester using an aliphatic dicarboxylic acid as the dicarboxylic acid has been molded into sheet, film or fiber form for practical use.
One of the reasons why the above mentioned polyester has not been practically used is that even if the aliphatic polyesters have crystallinity, most melting points of the above-mentioned aliphatic polyesters are 100.degree. C. or lower and they also have poor heat stability when they are melted. Of further importance is that the properties, particularly mechanical properties such as tensile strength, of these aliphatic polyesters are of an extremely low level; the polyester having the same level of number-average molecular weight as that of the above-mentioned polyethylene terephthalate shows much poorer properties, thus practical uses have not been found at all for these aliphatic polyesters.
It seems that the studies to improve the physical properties of the aliphatic polyesters by increasing their number-average molecular weights have not made enough progress because of their poor heat stability.
Polyesters such as polyethylene terephthalate or the like, which are largely for general use in the fields of packaging containers and industrial materials at present are not biodegradable by microorganisms, and therefore remain forever without degrading if simply disposed of after use, thus causing the pollution of rivers, oceans and soil. In order to prevent such a problem, polyesters need to be incinerated. However, the amount of their combustion heat, that is 5,500-6,000 kcal/kg, is greater than the average value of the combustion heat of municipal refuse, though it is lower than polyethylene and polypropylene, and this causes a large amount of wear on incinerators.
An object of the present invention is to provide an aliphatic polyester resin composition which has sufficiently high molecular weight for practical use, and has excellent heat stability and mechanical properties typically represented by tensile strength, and which it easily disposed of because it has a low combustion heat and is able to be decomposed by microorganisms or the like. Further, the present invention is aimed at providing an aliphatic polyester resin composition that has intrinsic heat seal characteristics by nature, as well as providing a sheet obtained by such a composition and articles formed thereof.